Decided not to install a seat post bag ..probably will have to find another alternative to store my tools and other small items on the RadExpand.

I was not able to install the Topeak Explorer rack due to the wide frame of RadExpand 5 - ended up returning it. Was pretty bummed it did not work - was very excited to remove the the Rad Rack and replace with the Topeak MTX system.

Still on the lookout for a solid rear bag that would be a secure fit on the rad rear rack but also easy to detach to carry my camera equipment.

I checked your post again and I actually tried the medium size of the bag instead of the large - https://amzn.to/3keCq20

Is it possible for you to explain or provide closer pics of how you adapted the straps to tie it down to the rad rack ?

Thank you ..I will have to look into how you attached it ..and see if that works for me too.
What bag did you end up buying - asking esp. because the straps look different on your bag as compared to what I have seen..

Following along this interesting topic..

Damn this adaptor is pricey  https://www.cascademanufacturing.com/products/rad-topeak-bag-adapters - $120/- plus shipping.

Following along..

I have spent hours tyring to find the same solution reading about Topeak Racks and Adaptors...

I wished this would have worked - https://www.topeak.com/us/en/products/248-Accessories/1323-OMNI-QUICKTRACK-ADAPTER but it too narrow to fit on the rad rack as per my reading.

The cascade made adaptor is currently out of stock

Following along..

I tried adding the ROCK BROS Bike Rack Bag Trunk Bag (https://amzn.to/3OwofTU) on top of the Rad Rack that came with the Rad Expand5 and the straps don't align well to keep the bag secure.

I really like the Topeak MTS system and convinced myself to replace the Rad Rack with the Topeak Explorer Rack. I don't know if it will fit or how the fit will be but someone mentioned in a review that it worked on a Radmini.

Are bike seat bags not supposed to be installed if using a suspension post ?
I tried a few highly rated bags but none was a good fit on this post..the straps were not in the right place to secure the bag. I am using this suspension post along with A Cloud-9 seat on a Rad Expnd 5.